The present invention relates to a film of fiber-reinforced regenerated cellulose which is impermeable to gas, a process for producing the film, and a tubular casing, preferably a sausage casing, produced from the film and which has a sealed seam running along the longitudinal axis of the casing.
U.S. Pat. No. 4,401,136 discloses a tubular casing, in particular, a sausage casing, produced from a film of the present type. The film web is shaped into a tubing and the edge regions, which are brought close to each other, are joined by means of a film strip by sealing the film strip against the gas-impermeable sealable surface layer of the film web. Due to its oxygen-, water-, and water vapor-impermeable layer this tubular casing can, for example, be employed as a sausage casing for emulsion-type and cooked sausages.
However, it has been found that problems arise when a longitudinal seam is formed by sealing the edge regions with a film strip. In cases where an overlapped seam is formed from the known film material, the seam obtained is relatively weak and tends to tear.
There has also been described in German Offenlegungsschrift No. 29 35 080, a tubular casing which, in addition to a gas-impermeable internal coating of a vinylidene chloride copolymer, possesses a gas-permeable external coating of a vinylidene chloride copolymer. This tubular casing is seamless, so that the basic problems to be solved by the present invention do not arise. The external, gas-permeable coating is applied in order to improve the shock resistance of the known casing when it is used as a sausage casing, and to prevent tearing of the sausage casing when the sausage is cut.
Such seamless tubings of fiber-reinforced regenerated cellulose are produced by a completely different process than the seamed casings to which the present invention is directed. In the preparation of a seamless casing, a non-woven fiber web is shaped into a tubing having overlapping edges which are glued with viscose. After single- or double-sided coating of the tubular-shaped non-woven fiber web with viscose by means of annular slot dies, the tubing is coagulated, regenerated into cellulose hydrate gel, and dried. During this process the tubing is internally supported by the gas mixture which forms during coagulation and regeneration and by an enclosed volume of air which is introduced during drying. In an optional step, the tubing is stretched in the longitudinal and transverse directions to improve its mechanical properties.
A disadvantage of the above process is that the laid-flat gel tubing exhibits weakened zones, which are referred to as extrusion edges, in the area of the edges running along the longitudinal axis of the laid-flat tubing. The stretching process also presents disadvantages. Neither the internal pressure exerted by the process gases, nor the internal pressure during drying can be controlled very precisely, so that it is impossible to keep the diameter of the tubings constant within narrow limits. A further disadvantage is that, in the case of stretching by means of expanding gases, there is a fixed ratio of the stretching forces acting in the tangential and radial directions. Accordingly, the mechanical properties of the seamless tubing, and, in particular, the shrink properties and strength values in the longitudinal and transverse directions, are fixed, as well.
The internal coating comprised of, for example, a material which is impermeable to water and water vapor, which is necessary if the tubing is to be used as a sausage casing for cooked and emulsion-type sausages, is applied to the inside of the seamless tubing in the form of a solution or dispersion. This requires a technically complicated process which is susceptible to disturbances. For example, the solvent or dispersing agent must diffuse through the tubing wall to the outside surface of the tubing in a lengthy process stage. Furthermore, it is difficult to control the film formation on the inside wall of the tubing. Finally, the mechanical properties, adjustment of which has been accomplished in a complicated process during the production of the tubing, are often impaired by the coating process. Since the tubings have to be laid flat immediately after coating, there exists the danger of damaging or weakening the water vapor-tight coating in the area of the squeezing edges. Due to these coating faults, the packed foodstuffs spoil or take on an unattractive appearance, as a result of water loss or undesired access of oxygen.
In order to avoid these disadvantages of seamless tubings of fiber-reinforced regenerated cellulose, the present invention is concerned with tubings having a sealed seam, which are produced from a flat film web, more along the lines of the tubings discussed in U.S. Pat. No. 4,401,136.